Help! Sick holly. Leaves turned bright orange

Hi

Any advice very welcome.... please!

I have a holly ...possibly golden king /golden queen?  think it is a male as it has never had any berries. (golden queen is actually a male?? so I am told)

It is well established - possibly 10 years old or so. It is around 10 feet tall.

It had lovely variegated green/lime/yellow leaves and they have suddenly turned ver y bright orange all over the entire leaf starting at the top of the shrub. They are starting to fall off and a couple of small branches look as though they are dying.

It is next to a wall, in a fairly sheltered situation, part sun, part shade. it is next to a magnolia that is doing very well. I have not fed the magnolia and as far as I know the soil is neither very acid or very alkaline. The soil is not boggy. It tends to be drier rather than damp. I have looked for signs of little beasties on the leaves but there is nothing. 

I watered it fairly well a month ago....could this be the problem? I also gave it a very small helping of potash ....have I done the wrong thing and is it "curtains" for the lovely tree/shrub? Can I blame it all on the magnolia?

Many thanks